The invention relates to a photocurable, negative-working mixture and a recording material, obtained therefrom, which is suitable for the production of elastic relief printing plates.
Mixtures of the said generic type are known and usually contain an elastomeric binder, a compound which can be polymerized by free radical polymerization and a polymerization initiator which can be activated by actinic radiation. After exposure, mixtures of this type, which, for example, are described in DE-C 2,215,090, can be developed only with organic solvents to give a relief image.
JP-A-246047/87 describes a system which can be developed in an aqueous medium. This system is based on partially saponified polyvinyl acetate and contains a polyether polyurethane having mercapto groups as a graft base. Since systems with vinyl acetate which contain mercapto groups have very high chain transfer constants, grafting in such systems takes place expressly on the mercapto groups.
It is true that in U.S. Pat. No. 4,042,386 and DE-B 1,902,639, 1,917,917 and 2,402,808 photosensitive recording materials which are based on partially saponified polyvinyl acetate and can be developed in an aqueous medium are described for the production of relief printing plates, but an addition of water is needed for homogenization and layer formation, which makes a subsequent drying process necessary. Furthermore, these relief printing forms do not have the high elasticity and low Shore A hardness necessary for flexographic printing.
In DE-A 3,541,162 and EP-B 0,080,664 internally plasticized, partially saponified polyvinyl acetates are described as binders for photosensitive recording materials. These graft polymers, which are obtained by polymerization of vinyl esters in the presence of polyalkylene oxides and subsequent partial saponification, are indeed more elastic than pure partially saponified polyvinyl acetates, but the addition of water is likewise necessary when processing the corresponding photopolymer mixtures into sheet-like recording materials.